The organic light-emitting display is a promising candidate for the display of the next generation. Nevertheless, elements of the organic light-emitting display reflect ambient light easily, so that the display has lower contrast and inferior performance. Additionally, the conventional cathode of the organic light-emitting display is made of Ca, Mg or LiF, which are highly active and easy to be oxidized or to react with the organic layer. Consequently, the conventional organic light-emitting display deteriorates easily and has a shorter lifespan.
Many solutions to the low contrast problem of the organic light-emitting display have been proposed in prior art, such as adhering a polarizer to the emitting side of the organic light-emitting display to improve the contrast. However, the polarizer would cut the brightness down to 30% of the original value, and then force the operation voltage to be higher to achieve target brightness, which results in shorter lifespan of the display.
Dobrowolski et al. disclosed an “Optical Interference, Electroluminescent Device having Low Reflectance” in U.S. Pat. No. 5,049,780. A semi-transparent thin metal layer, a transparent conductor layer (e.g. ITO) and a reflector layer are sequentially coated on the organic light-emitting display. Taking advantage of the light interference, the phase difference between the first and the second reflections of the ambient light is made to be 180°, and so the contrast is elevated. This method may have the brightness larger than 50% of the original value. However, this multi-layer structure is difficult to fabricate, since it requires accurate film thickness control and adopts co-evaporation process.
EPO Patent publication No. EP1164817 “Outdoor electroluminescent display device” discloses a method of coating a reflector layer on the emitting region of the outside of the display and an absorber layer on the non-emitting region, to reduce the reflection from the non-emitting region and elevate the contrast. This method losses less brightness, but requires precise pattern process. The fabrication of the organic light-emitting display cannot adopt the photolithography process, and the pattern has to be formed with metal mask instead. Therefore, this method is difficult to practice.
Besides, the above-mentioned methods only improve the contrast, but do not resolve the problems of reaction with the organic layer and the oxidation of the cathode.